A British exit from the European Union (EU) would be "catastrophic", businessman Sir Richard Branson has said.
The founder of Virgin Group said that Britons should be "proud" of being Europeans and remaining a member of the EU meant "talking from a position of strength".
Speaking on BBC One's The Andrew Marr Show, the adventurer and philanthropist said: "The wonderful situation we have at the moment where there's this enormous trading bloc, you can treat it just as if it's the UK.
"Trade anywhere within Europe and so how anybody would want to go back to the days where all these barriers were put up, I just find it inconceivable."
Sir Richard argued that "the whole of the EU administration put together has no more people than the number of people that run a city like Birmingham".
Britain, he said, had a trading bloc equivalent to the size of the US and was on an "equal footing" when negotiating trade or airline deals.
He said: "We're talking from a position of strength. If we go back to being Great Britain again we will have our hands tied behind our back."
Asked about the proposal to remove some benefits such as tax credits from people from other parts of the EU to prevent higher immigration into the UK, he said: "If they need to tinker with one or two of these things in order to make sure that we don't end up doing something more catastrophic like leaving Europe, that's fine by me.
"I think there are much bigger issues that I'm concerned about."
Sir Richard argued: "We're the first generation that haven't been to war in Europe and this to me is even more important, by being part of a big trading bloc.
"We spend time with each other, we marry each other, we live in each other's countries, we're not going to go to war with each other."
Asked if he was pleased Britain was staying out of the euro, he replied: "Not particularly, I think that if we were part of the euro right now our currency would be a lot cheaper, Great Britain would be doing that much better in trading in Europe because the pound is a lot stronger than the euro, it makes it more difficult for us."
He added: "I'm very happy just to say why we should be proud of being Europeans.
"Each European country is trying to improve Europe, that's much better to do from within Europe than from without."
Labour former home secretary Alan Johnson, who will lead the party's campaign for a Yes vote in the referendum, criticised the way David Cameron was attempting to renegotiate Britain's relationship with the EU.
He told BBC One's The Andrew Marr Show: "I think the main issue here is he's dealing with reform as if it's an event, whereas it's a process, it's a continuous process and as someone who has negotiated most of my adult life, this is not the way to actually negotiate."
He added: "I hope, I am like, I think, most of the British people, I am not a zealot about Europe, I have a quiet, probably understated view that being in Europe is in our best interests for the future, for our standard of living, for our quality of life, for our future, particularly for young people.
"So, I think most British people when they focus on the arguments here, it won't be about this soap opera of what the Prime Minister is doing in Europe, it will be about a much longer-term issue, about do we want to be in isolation or do we want to be on the field playing the game?"
On sharing platforms, Mr Johnson said: "I think people who come from different political persuasions but have the same ideals shouldn't be afraid to share platforms exactly as they did in '75."
Mayor of London and Tory MP Boris Johnson may call for a "no" vote in the EU referendum to prompt Brussels to offer a better deal, which the public could then back in a second referendum, according to reports in The Sunday Times.
Former chancellor and veteran Conservative MP Ken Clarke described the notion as "nonsense".
He told Sky News's Murnaghan programme: "What on earth does he mean by this? It can't be yes one week and no the next, or people will get deeply suspicious of exactly where he is.
"The idea that you reject anything that's offered the first time round in the fond imagining that more of these extreme nationalists will get what they want in the second time and then you can shut it all up is nonsense."
He added: "It's no good thinking that you can have a sensible negotiation and then have a referendum where the public reject it and then suddenly they will cave in and offer us all kinds of things which will be destruction of the EU if they gave them."
Mr Clarke said Britain should concentrate on the "sensible" things Mr Cameron set out in his first Bloomberg speech such as completing the single market, trade deals, serious deregulation and not doing things in Brussels which could perfectly well be done in capitals.
He added he was "deeply opposed" to "discriminating in your tax and benefits system against foreigners".
Mr Cameron earlier insisted he is planning a "wholly positive" campaign in the EU referendum, after a leaked memo suggested he might seek to play on voters' fears by stressing the risks of a British exit.
Speaking at the conclusion of a European Council summit on Friday at which he secured agreement to start talks on renegotiating the terms of British membership, Mr Cameron acknowledged he did not have the backing of all 27 fellow leaders but insisted he was "confident" of reaching a deal allowing him to recommend a Yes vote in the poll, promised by the end of 2017.
Mr Cameron declared himself "delighted" with the agreement that behind-the-scenes talks will begin shortly between UK and EU officials, with national leaders returning to the issue at a Council summit in December.
After fellow EU leaders voiced reservations about his call for treaty change to reform Britain's membership, Mr Cameron acknowledged that renegotiation will be a tough process.